This invention relates to ophthalmological surgery techniques which employ a laser to effect ablative photodecomposition of the anterior surface of the cornea in order to correct vision defects.
Ultraviolet laser based systems and methods are known for enabling ophthalmological surgery on the surface of the cornea in order to correct vision defects by the technique known as ablative photodecomposition. In such systems and methods, the irradiated flux density and exposure time of the cornea to the ultraviolet laser radiation are so controlled as to provide a surface sculpting of the cornea to achieve a desired ultimate surface change in the cornea, all in order to correct an optical defect. Such systems and methods are disclosed in the following U.S. patents and patent applications, the disclosures of which are hereby incorporated by reference: U.S. Pat. No. 4,665,913 issued May 19, 1987 for xe2x80x9cMETHOD FOR OPHTHALMOLOGICAL SURGERYxe2x80x9d; U.S. Pat. No. 4,669,466 issued Jun. 2, 1987 for xe2x80x9cMETHOD AND APPARATUS FOR ANALYSIS AND CORRECTION OF ABNORMAL REFRACTIVE ERRORS OF THE EYExe2x80x9d; U.S. Pat. No. 4,732,148 issued Mar. 22, 1988 for xe2x80x9cMETHOD FOR PERFORMING OPHTHALMIC LASER SURGERYxe2x80x9d; U.S. Pat. No. 4,770,172 issued Sep. 13, 1988 for xe2x80x9cMETHOD OF LASER-SCULPTURE OF THE OPTICALLY USED PORTION OF THE CORNEAxe2x80x9d; U.S. Pat. No. 4,773,414 issued Sep. 27, 1988 for xe2x80x9cMETHOD OF LASER-SCULPTURE OF THE OPTICALLY USED PORTION OF THE CORNEAxe2x80x9d; U.S. patent application Ser. No. 109,812 filed Oct. 16, 1987 for xe2x80x9cLASER SURGERY METHOD AND APPARATUSxe2x80x9d; and U.S. Pat. No. 5,163,934 issued Nov. 17, 1992 for xe2x80x9cPHOTOREFRACTIVE KERATECTOMYxe2x80x9d.
In the above-cited U.S. Pat. No. 4,665,913 several different techniques are described which are designed to effect corrections for specific types of optical errors in the eye. For example, a myopic condition is corrected by laser sculpting the anterior corneal surface to reduce the curvature. In addition, an astigmatic condition, which is typically characterized by a cylindrical component of curvature departing from the otherwise generally spherical curvature of the surface of the cornea, is corrected by effecting cylindrical ablation about the axis of cylindrical curvature of the eye. Further, a hyperopic condition is corrected by laser sculpting the corneal surface to increase the curvature.
In a typical laser surgical procedure, the region of the anterior corneal surface to be ablated in order to effect the optical correction is designated the optical zone. Depending on the nature of the desired optical correction, this zone may or may not be centered on the center of the pupil or on the apex of the anterior corneal surface.
The technique for increasing the curvature of the corneal surface for hyperopia error correction involves selectively varying the area of the cornea exposed to the laser beam radiation to produce an essentially spherical surface profile of increased curvature. This selective variation of the irradiated area may be accomplished in a variety of ways. For example, U.S. Pat. No. 4,665,913 cited above discloses the technique of scanning the region of the corneal surface to be ablated with a laser beam having a relatively small cross-sectional area (compared to the optical zone to be ablated) in such a manner that the depth of penetration increases with distance from the intended center of ablation. This is achieved by scanning the beam more times over the deeper regions than the shallower regions. As pointed out in U.S. Pat. No. 5,163,934, such ablations tend to be rougher than area ablations. The result is a new substantially spherical profile for the anterior corneal surface with maximum depth of cut at the extreme outer boundary of the optical zone. Another technique disclosed in the above-cited U.S. Pat. No. 4,732,148 employs a rotatable mask having a plurality of elliptical annular apertures which are progressively inserted into the laser beam path to provide progressive shaping of the laser beam in order to achieve the desired profile.
One of the major difficulties encountered in the application of laser surgery techniques to effect hyperopic refractive error corrections lies in the nature of the boundary between the optical zone and the untreated area. Since the anterior surface of the cornea is sculpted during the process to have an increased curvature, the maximum depth of cut necessarily occurs at the outer boundary of the optical zone. The generally annular region between this outer boundary and the adjacent untreated anterior surface portion of the cornea typically exhibits steep walls after the completion of the photoablation procedure. After the surgery the tendency of the eye is to eliminate these steep walls by stimulated healing response involving concurrent epithelial cell growth and stromal remodelling by the deposition of collagen, which results in corneal smoothing by filling in tissue in the steep walled region. This natural healing response acts to eliminate the discontinuity, resulting in a buildup of tissue in the steep walled region and over the outer portion of the optical zone. This natural phenomenon, sometimes termed the xe2x80x9chyperopic shiftxe2x80x9d in phototherapeutic keratectomy, causes a lack of precision for a given surgical procedure and diminished predictability, which tend to counteract the beneficial effects of the refractive correction procedure and thereby reduce the desirability of the procedure to the prospective patient.
Efforts have been made in the past to laser sculpt a transition zone to provide a more gradual sloping of the walls and to eliminate the sharp discontinuity between the outer edge of the optical zone and the edge of the untreated area. Efforts have included the use of a beam rotation or scanning mechanism operated by a computer to provide programmed ablation of the transition zone to achieve a sigmoidal or other profile. While somewhat effective, these efforts suffer from the disadvantage of typically requiring additional optical elements (such as a rotatable off-axis mirror or revolving prism having suitable optical properties) which adds complexity to the delivery system optics commonly found in laser sculpting ophthalmological surgical systems. One specific technique, which is disclosed in published European Patent Application No. 0 296 982 published Dec. 28, 1988, employs a rotatable mask having one or more profiling apertures whose shape is designed to provide a smoother profile in the transition zone in the course of performing a specific ablation procedure. This reference also teaches the use of a rotating prism aligned along the beam axis in combination with a translatable platform bearing a focusing lens in order to both translate and rotate the aperture image along the anterior corneal surface. This technique, while considered effective for some purposes, requires a relatively complicated optical delivery system in order to provide the desired profiling. In addition, the use of mirrors and prisms in delivery system optics in laser surgery systems suffers from certain disadvantages. In particular, the addition of prisms decreases the total energy transmission of the system. Further, the reflectance of dielectric mirrors used in certain systems varies with reflectance angle, which can dynamically alter the irradiance delivered to the cornea while displacing the beam image over the cornea.
Another difficulty encountered in the application of laser surgery techniques to effect hyperopic refractive error corrections lies in the requirement for relatively large transition zones outside the optical zone. In particular, while the intended optical zone is typically on the order of about 5 mm in diameter, the outer limit of the transition zone can be as large as 10 mm in diameter. If the rotating mask arrangement described above is used to effect the ablation in both the optical zone and the transition zone, the beam diameter must be commensurate in size with the largest aperture outer diameter (i.e., at least about 10 mm). In general, the larger the beam diameter the less uniform the energy density across the beam and the less reliable the photoablation process. Further, the increased beam area requires a laser beam of substantially greater energy, which necessitates a more expensive laser. Also, the increased energy flowing through the optical components causes optical deterioration at a faster rate, thereby increasing maintenance and replacement costs. Another disadvantage inherent in a rotating mask system is that the resulting ablation frequently exhibits a central ablation surface which is rougher than desired when a hyperopic correction is conducted.
The invention comprises a method and system for performing ablative photodecomposition of the corneal surface which is capable of providing relatively smooth transition zones along with accurate sculpting of the anterior or other corneal surface to effect symmetric or asymmetric refractive corrections requiring relatively large area coverage. The invention is further capable of smoothing the corneal surface after a refractive correction has been ablated, and is further effective in performing phototherapeutic keratectomies. The invention uses a laser beam of smaller beam size than known devices, and can be readily designed into new ophthalmological surgery systems or retrofitted in existing ophthalmological surgery systems.
From a method standpoint, the invention comprises the steps of directing a laser beam toward a variable aperture, profiling the beam with the variable aperture to produce a variable area profiled beam, and scanning the profiled beam over a predetermined area of a corneal surface of an eye while varying the profile in a predetermined manner. The step of profiling can include the alternative steps of intercepting the laser beam with a variable width slit or a variable diameter diaphragm, or both; and the step of scanning may include the step of selectively varying the slit width, the diameter of the diaphragm, or both. During scanning, an axis of rotation for the profiled beam may be established and the profiled beam is radially displaced from the axis of rotation by a preselected amount during scanning. The angular position of the profiled beam about the axis of rotation may also be varied in a predetermined manner during scanning.
Various corrective procedures can be performed according to the method of the invention. In a first procedure, the scanning is performed by scanning the beam over successive arcuate or annular bands in the predetermined area of the corneal surface. In another procedure, the profiled beam is scanned over a predetermined portion of the area of the corneal surface while alternately enlarging and reducing the size of the variable aperture. The predetermined portion of the area may comprise a central zone of the corneal surface or an outer region of the corneal surface.
The step of scanning may be preceded by the steps of establishing an optical zone on the anterior corneal surface in which the desired refractive correction is to be effected, the optical zone having an outer boundary, and establishing a transition zone between the optical zone and the remaining anterior corneal surface. After establishing the optical zone and the transition zone, the scanning step is performed by scanning the profiled beam over the optical zone and the transition zone. The transition zone has an inner boundary and an outer boundary, and the step of profiling the beam may be conducted by intercepting the beam with a variable diameter diaphragm and a variable width slit having inner and outer edges, and the step of scanning is performed by maintaining that portion of the profiled beam corresponding to the intersection of the diaphragm and the outer edge of the slit adjacent the outer boundary of the transition zone. During scanning, the slit width can be narrowed by translating the inner edge of the slit toward the outer edge.
The step of scanning may be preceded by the steps of creating a treatment table containing a listing of coordinate references for the profiled beam and the number of laser pulses at each coordinate reference required to effect the desired refractive correction, and sorting the listings in the treatment table to establish a scanning pattern for the profiled beam.
From another method aspect, the invention includes the step of directing a laser beam along a path, profiling the beam with a variable aperture to produce a profiled beam, establishing an axis of rotation, displacing the profiled beam from the axis of rotation, and varying the angular position of the profiled beam about the axis of rotation to cause the beam to describe a path about a center of rotation corresponding to a desired ablation center. The step of profiling the beam may be performed by intercepting the laser beam with a variable aperture, such as a variable diameter width slit or a variable diameter iris diaphragm or both, and varying the aperture size in a predetermined manner while varying the displacement of the profiled beam in a manner related to the slit width. In a preferred implementation of the method, the steps of displacing the profiled beam and varying the angular position of the profiled beam are performed with an imaging lens by radially displacing the lens from the path and rotating the lens about the center of rotation.
To effect a predetermined hyperopic refractive correction, the method comprises the steps of directing a laser beam along a path, and selectively irradiating the corneal surface of the eye to ablate the appropriate contour required to effect the hyperopic refractive correction by intercepting the beam with a variable width slit to produce a profiled beam having an initial width, displacing the profiled beam exiting the slit by an initial amount from the axis of rotation, rotating the slit by a predetermined angular amount about the axis of rotation, adjusting the slit width, displacing and rotating the profiled beam exiting the slit by selected amounts, and repeating the steps of rotating the slit, adjusting the slit width and displacing and rotating the profiled beam until the hyperopic correction is completed. The step of displacing the profiled beam exiting the slit is preferably performed such that the edge portion of the exiting profiled beam associated to a first slit edge initially impinges the optical zone adjacent the center and the edge portion of the exiting beam associated to a second slit edge impinges the desired transition zone adjacent the outer edge. According to this method, the edge portion of the exiting profiled beam associated to the first slit edge impinges the optical zone at progressively increasing distances from the center and the edge portion of the exiting profiled beam associated to the second slit edge impinges the transition zone adjacent the outer edge. Preferably, the step of displacing and rotating the profiled beam by selected amounts is performed with an imaging lens positioned between the slit and the eye by first displacing and rotating the lens from a starting position, pulsing the laser and then rotating the lens to a subsequent angular position, which is preferably the existing position plus a predetermined incremental amount.
From an apparatus aspect, the invention comprises an ophthalmological surgery system for performing selective ablation of the corneal surface of an eye to effect a desired refractive correction, the system comprising means for directing a laser beam along a path, variable aperture means for profiling the beam to produce a variable area profiled beam, and means for scanning the profiled beam over a predetermined area of the corneal surface while varying the profile in a predetermined manner. The variable aperture profiling means preferably comprises a variable width slit and means for selectively varying the slit width during scanning. Alternatively, the variable aperture profiling means may comprise a variable diameter diaphragm, preferably an iris diaphragm, and means for selectively varying the diameter of the diaphragm during scanning. The scanning means includes means for radially displacing the profiled beam from an axis of rotation by a preselected amount, and means for varying the angular position of the profiled beam about an axis of rotation in a predetermined manner. The scanning means preferably includes an imaging lens positioned in the path of the profiled beam and means for displacing and rotating the lens means with reference to an axis of rotation. For the preferred embodiment in which the variable aperture profiling means includes a variable width slit and means for rotating the slit, the means for displacing and rotating the lens means and the means for rotating the slit are operationally coupled.
The system and method can be incorporated into existing laser surgery systems having a variable diameter iris aperture and a variable width slit mounted on a rotatable platform by modifying the mounting mechanism used for the existing imaging lens to enable the lens to be translated radially of an axis of rotation and rotated with the slit platform about the axis of rotation. The invention is capable of providing wider area beam coverage of the corneal surface with a laser having a conventional beam size, thereby eliminating any need for a larger beam laser and providing wider area coverage with lower energy requirements than many known devices.